Long strings of pipe are often used to bring up oil and gas deposits after a well has been drilled. These pipe strings consist of a number of pipe sections which have external threads at one end and internal threads at the other end. The pipe sections are screwed together at the well to form pipe strings. It is important that the threaded connections of the pipe sections form a perfect connection so that no leakage occurs at the joints. In order to assure that the threaded connections are undamaged and that a good connection can be formed, each pipe section is rack tested prior to use at the well. This testing also insures adequate pipe body strength. During a rack test, the pipe sections are placed on a rack, the ends are sealed with test plugs, and the pipe subjected to hydrostatic pressure to detect leakage at the threaded joints.
In spite of the fact that rack testing is very important, a number of problems still exist in this area, especially when a hydraulically operated machine is used to make up the test plugs with the ends of the pipe. Occasionally piping becomes bent. This makes it difficult to align the test plug with the pipe end. If the test plug and the pipe end are not properly aligned during the make-up, cross-threading or side loading can occur.
In addition, debris or foreign matter may become lodged between the threads during make-up resulting in damage to the threads.
In attempting to bring a test plug into more perfect alignment with a pipe end, prior art rack testers have utilized a test plug which is mounted onto a unit with springs or air bags which allows the test plug to undergo limited movement relative to the pipe and thus move into vertical alignment with the pipe end prior to make-up. Although mounting the test plug in this manner decreases the probability that thread damage will occur, the use of springs or air bags has certain drawbacks. Where such springs or air bags are used, the components are limited to only one fixed resistance. This controls the amount of movement which the test plug may undergo. Therefore, in order to vary the resistances and alter the height to which the test plug may move, in accordance with the condition of the pipe end or the size or weight of the pipe, the mechanical springs must be changed out in order to change the effective spring rate. Similary, the air pressure must be increased or decreased in the air bags.
Further, the heavier the test plug assembly is, the higher the effective spring rate or fixed resistance must be. Higher resistances can overload pipe threads and cause thread damage if the test plug deflects in order to align with the pipe. On the other hand, lower resistances may not be sufficient to withstand the torques that are applied to the pipe end during the threading of the test plug into the pipe end.
In order to compensate for this deficiency, several prior art techniques have been developed which involve apparatus or methods for monitoring the making up of threaded tubular joints in order to detect and control the amount of torque applied to the joint. Using these techniques, one can detect and control the excessive torque which can damage threads. One such apparatus and method is shown in U.S. Pat. No. 3,096,643, which discloses a combined screw torque and tension measuring or torque detecting device for threaded fasteners. This device is functionally adapted to rapid repetitive testing of small diameter elements and would not be adaptable for use with a pipe testing apparatus for large diameter pipe and casing. U.S. Pat. No. 4,176,436 also discloses a control means for an apparatus for assembling threaded members such as pipe sections to form a pressure sealed joint. The apparatus produces a warning of a bad joint upon the measurement of a predetermined torque prior to reaching a minimum number of turns and vice versa. Another patent, U.S. Pat. No. 4,259,896, discloses an apparatus and method for inspection and control of threaded joint tightening processes by registering during the main part of the tightening process the momentary values of the torque delivered to the joint and the angular displacement of the joint. This is accomplished by comparing these values in a control unit with limit values fo a narrow check range so that a wider control range damage to the threads may be prevented.
The major disadvantage of these prior art systems is that it is not possible to detect slight misalignments of the test plug with the pipe end and/or very small changes in torque. As a result, threads may be damaged before the operator is alerted to the fact that misalignment of excessive torque has occurred. In order to solve these problems the apparatus of the present invention has a variable spring rate which permits proper alignment of the threaded members and which is capable of determining when thread damage is occurring during make-up by detecting misalignment of the test plug with the pipe end and/or very small changes in torque.